1. Field of the Invention
The invention pertains to medical ultrasound imaging systems which utilize an array of transducer elements to transmit ultrasound energy. In particular, it pertains to an apodization function during transmission of a focused beam of coherent ultrasound energy from an array of transducer elements to control the amplitude of the pulses transmitted from each transducer element. Reduced sidelobe levels and improved directivity are achieved through apodization at the transmit stage.
2. Description of the Prior Art
A typical ultrasound imaging system used for medical imaging includes at least one ultrasound transducer, usually in the form of an array of transducer elements. The transducer is usually connected to transmitter stage circuitry, receiver stage circuitry and whatever devices may be necessary to effect mechanical scanning of the transducer. The transmitter stage comprises a generator of electrical excitation signals which are sent to the transducer. The transducer converts these signals into periodic pulse trains of ultrasound mechanical energy.
Transducer arrays used in medical imaging systems come in many forms, such as linear arrays, phased arrays, annular arrays and collimated image arrays. A transducer array usually consists of a plurality of transducer elements disposed on a surface in some designed arrangement such as a row, matrix or other geometric pattern. The individual elements or groups of elements are actuated or pulsed in sequence to transmit a beam of ultrasound energy at a target. Ultrasound echoes are returned from the target and may be received on the same transducer elements. The pulse echo data received is then interpreted and displayed to produce an image.
In the technology of medical imaging with ultrasound, the principles of the linear array and the phased array are well known, though the technical terminology can be semantically misleading. For the purposes of this application, the following definitions will be used. A linear array is an electronically scanned array of similarly sized and shaped elements arranged in an extended line, side by side. A group of contiguous transducer elements are electronically selected from the extended array, are pulsed for transmission and then sometimes used for reception of resultant echoes. The selected group of transducer elements is then commutated one or more positions along the array and the process repeated to scan successive parallel regions in the body. A linear array projects beams of ultrasound energy perpendicular to the face of the transducer element. The image format is usually rectangular.
A phased array refers to a short linear array of transducer elements, the transmitted energy being deflected from the normal perpendicular beam by inserting delays in the pulse signal to each element. Similarly the received pulse echo is steered in angle by inserting delays in the signal path from each angle before summation. The resulting image is pie-shaped and accomplishes the so-called sector scan.
Electronic focusing of both types of arrays is possible by a different set of time delays. By introducing time delays, focusing or phasing is possible to improve lateral resolution over a particular depth range inside a target. Focusing improves lateral resolution (beam width) in the focal zone and it improves sensitivity because of higher energies produced in the focal area.
Most modern pulse echo ultrasound imaging systems utilize a linear transducer array and employ dynamic electronic focusing during signal reception. Most linear array transducer systems also employ a fixed focus during pulse transmission, using a fixed mechanical lens. The axial resolution of such an array depends on the length of the pulse. Resolution in the transverse plane depends on the elevational dimension of the array.
The prior art has made some attempts to introduce horizontal (longitudinal) focusing at the transmitter stage of an ultrasound imaging system. In the transmit mode time delays have been used to steer beams by constructive and destructive interference of all pulsed signals. Whether in the transmit mode or the receive mode, these prior art time delays are frequently implemented in analog circuits, which trigger an oscillator, or by the analog delay of an existing pulse train. However, the use of analog delays introduces jitter in transmission, cross-talk and other forms of time noise.
Related to the problem of focusing, the issue of sidelobe suppression has also been the subject of much attention in the prior art. For medical imaging applications where human tissue acts as a diffusely reflecting structure, it is important to be able to differentiate subtle tissue structures in the presence of strong reflectors. This ability is limited by the sidelobes of the point spread function. It is, therefore, of great interest to develop systems with small sidelobe levels.
The conventional technique for reducing the sidelobe levels of a transducer array is "Aperture Apodization". Aperture weighting functions, such as Gaussian or Hanning functions, are applied to signals on the array elements. Such techniques are described, for example, in Peterson et al "Quantitative Evaluation of Real-Time Synthetic Aperture Acoustic Images", in: A Review of Progress in Quantitative Nondestructive Evaluation (Plenum Press, New York, Vol. 1, 1982, pages 767-776) and in 't Hoen "Aperture Apodization to Reduce the Off-axis Intensity of the Pulsed-mode Directivity Function of Linear Arrays", Ultrasonics, September 1982, pages 231-236, which are incorporated herein, by reference, as background material. Using these prior art techniques, the sidelobe level can be reduced at the expense of some increase in main lobe width. This trade-off is a fundamental one which is also encountered in spectral analysis and antenna design. When energy under the sidelobes is reduced, more energy is introduced under the main lobe.
The peak pressure in the emitted ultrasound beam is related to the grey-level distribution in the resultant image. The cross-section of the ultrasound beam emitted by a transducer is described by the emission directivity function which at any distance from the transducer, is defined as the variation of peak pressure as a function of lateral distance to the beam axis. The directivity function of a transducer is used to characterize its spatial resolution as well as its sensitivity to artifacts.
The directivity function of a transducer is related to its aperture function (which is the geometric distribution of energy across the aperture of the transducer). The prior art has recognized that, in narrowband systems, the far-field directivity function corresponds to the Fourier transform of the aperture function; this relationship has been applied for beam-shaping in radar and sonar systems. This relationship does not hold true, however, in medical ultrasound systems which utilize a short pulse, and thus a broad frequency spectrum, and which usually operate in the near-field of the transducer. Therefore, in medical ultrasound applications the directivity function of a transducer must be rigorously calculated or measured for each combination of transducer geometry and aperture function. The directivity function of a transducer may, for example, be calculated on a digital computer using the approach set forth in Oberhettinger On Transient Solutions of the "Baffled Piston Problem", J. of REs. Nat. Bur. Standards-B 65B (1961) 1-6 and in Stepanishen "Transient Radiation from Pistons in an Infinite Planar Baffle", J. Acoust. Soc. Am. 49 (1971) 1629-1638. One applies a convolution of the velocity impulse response to the transducer with the electrical excitation and with the emission impulse response of the transducer.
A transducer may be apodized by shaping the distribution of energy applied across the transducer to a desired aperture function. For a single disc, piezoelectric transducer, this has been accomplished by shaping the applied electric field through use of different electrode geometries on opposite sides of the disc as described, for example, in Martin and Breazeale "A Simple Way to Eliminate Diffraction Lobes Emitted by Ultrasonic Transducers", J. Acoust. Soc. Am. 49 No. 5 (1971) 1668, 1669 or by applying different levels of electrical excitation to adjacent transducer elements in an array. However the method of Martin and Breazeale is limited to a number of simple aperture functions and the use of separate surface electrodes requires complex transducer geometries and/or switching circuits.